An RFID tag can be attached to a to-be-identified object, such as a product or a person, and it has been used for logistics management or entering/leaving management at a station or at an office. For example, an RFID tag may store attribute information of a to-be-identified product to which the tag is attached. The RFID tag can be used by a user to identify the attribute information or the current location of the product to which the tag is attached, when the user's wireless terminal accesses the RFID tag.
For example, the following wireless search device has been known as a means for searching for a current location of an active RFID tag. First, the wireless search device queries an RFID tag to transmit an identification (ID) number.
Subsequently, the wireless search device that receives a radio wave that is transmitted from the RFID tag estimates a distance and a direction from the wireless device to the RFID tag based on reception intensity of the radio wave, an arrival direction of the radio wave, and the like. Finally, the wireless search device associates the estimated location with the ID number of the RFID tag, and displays the estimated location that is associated with the ID number of the RFID tag. At this time, in order to scan in all directions for the direction in which the RFID tag is located, it may be required to vary the antenna directivity for receiving a signal over 360 degrees by manually changing the direction of the wireless search device over 360 degrees for each search operation, or by application of beam-forming technology.
In such a system that is for searching for the current location of an RFID tag, when a transmission interval of transmitting signals from the RFID tag is too large, performance of searching for the tag (e.g., search accuracy or a search success rate) may be degraded, and when a transmission interval of transmitting signals from the RFID tag is too small, battery energy consumption may be increased because of a likelihood of collisions by simultaneous transmissions from a plurality of tags and frequent transmissions. Thus, in the above-described RFID tag search system, a technique may be required that is for adjusting the transmission interval for transmitting the signals depending on a situation.
Patent Document 1 discloses a terminal location search device for locating a position of a mobile terminal based on a positioning signal from the mobile terminal, as a scheme for adjusting the transmission interval of transmitting the signals depending on a situation in the above-described RFID tag search system. Specifically, the terminal location search device may transmit a control signal for adjusting the transmission interval for periodically transmitting positioning signals to the mobile terminal depending on current moving velocity of the mobile terminal, and the mobile terminal that receives the control signal may transmit a positioning signal at every time interval that is indicated by the terminal location search device. Namely, the terminal location search device can prevent positioning accuracy from being degraded by adjusting the transmission interval for transmitting positioning signals at high moving speed to be smaller than the transmission interval for transmitting positioning signals at low moving speed.
As a technical effort to overcome the problem of the increase in likelihood of collisions by frequent simultaneous transmissions from the above-described plurality of radio wave sources, Patent Document 2 discloses a communication system and a base station for improving a throughput by preventing signals from interfering with each other between mobile terminals. Specifically, the base station apparatus monitors the number of the mobile terminals (users) that are located within its own cell. At the same time, in response to the number of the mobile terminals, the base station apparatus transmits, from an RF transmitter, control information (frame format identification information) to the mobile terminals that are located within its own cell. Here, the control information is for controlling a frame format of a RACH depending on the number of the mobile terminals. At this time, the base station apparatus may execute control so that, as the number of the mobile terminals within the cell is increased, an amount of overhead information that is included in a frame having a frame format that corresponds to the number is reduced. In this manner, the amount of the overhead information is reduced depending on the number of the users, and a RACH frame whose length is shortened by the corresponding reduction amount can be transmitted. Thus, even if the number of the users is increased, a likelihood of occurrence of collision between frames on the RACH can be reduced, and the throughput can be enhanced.
Further, in the invention disclosed in Patent Document 3, the tag search device may switch power control operation for radio transmission and reception, depending on whether the current operating state of the device itself is a search state for searching for a desired RFID tag, or the current operating state of the device itself is a communication continuation state in which the tag search device is communicating with the detected RFID tag. Specifically, for radio transmission and reception by the device itself, in the search state, the device switches to a power control mode that corresponds to high antenna output power, and in the communication continuation state, the device switches to a power control mode that corresponds to antenna output power that is lower than that.